This invention relates to a wireless communications system with receive polarity diversity and integrated, time delayed, transmit polarity diversity, and more particularly to a wireless communications system having improved xe2x80x9creverse linkxe2x80x9d and xe2x80x9cforward linkxe2x80x9d performance enabling extended range and substantially xe2x80x9cbalance linksxe2x80x9d.
Wireless communication systems generally include a base station for receiving and transmitting electromagnetic radiations and mobile stations disposed within the coverage area of the base station. The mobile stations transmit electromagnetic radiations to and receive such radiations from the base station, where several such base stations are generally linked together through base station controllers (BSC) and mobile switching centers (MSC) to provide a seamless communication link between a mobile station and a calling or called party.
Wireless communications are typically embodied within two bands. Those systems between approximately 850 and 950 MHz are referred to as cellular and those systems between approximately 1.8 and 2.0 GHz are referred to as personal communications systems (PCS). This invention relates to both cellular and PCS systems.
The coverage area, or range of base stations are generally limited by the receive noise figure (NF) of the base station and by the radiated power of the mobile station on the uplink and by the BSC; transmit effective isotropic radiated power (EIRP) and the NF of the base station on the down link. Presently, base station architecture utilizes an antenna array comprising a plurality of spaced apart radiating elements for transmission and a separate such plurality of radiating elements for reception.
The radiating elements are generally electrically conductive members disposed on a support and are generally spaced between three-fourths and one wavelength apart. The antenna elements are generally connected to a combiner via short transmission lines.
To improve base station performance, the receive antenna configuration generally comprises two columns to provide spatial diversity or a single orthogonally polarized column having two orthogonal polarization outputs to provide polarization diversity.
The transmit antenna elements are generally disposed on a support, and the distance between the transmit or send antenna elements is typically quite large, often a few meters.
In a wireless communications system, the signal path from a base station to the mobile stations is referred to as the xe2x80x9cforward linkxe2x80x9d, while the signal path from the mobile station to the base station is the xe2x80x9creverse linkxe2x80x9d. Wireless communication systems are limited by the capability of the mobile stations to receive information in the presence of interference. A key performance measure is the minimum ratio of signal power to interference power that permits acceptable communication quality.
A xe2x80x9cbalanced linkxe2x80x9d is defined as the condition where the mobile station receives the minimum acceptable signal power from the base station (forward link) at essentially the same distance as the base station receives the minimum acceptable signal power from the mobile station (reverse link). Tower mounted amplifiers typically improve signal strength on both the forward and reverse links, but the improvement is not necessarily symmetrical. Regulatory limitations on transmitted power often mean that more performance improvement is available on the reverse link than the forward link.
Cellular and PCS systems rely extensively on reflected or xe2x80x9cmultipathxe2x80x9d energy to maintain the communication link. These paths constantly change as the mobile station and/or objects in the immediate vicinity of the mobile station move. The result of this propagation environment is that various reflected signals combine, both constructively and destructively over time, to form a net received signal with wide fluctuations in strength (i.e. Rayleigh fading) at any point in the coverage area. As the signal strength from the base station decreases, due to either increased distance from the base station or signal absorption as the user moves deeper into buildings, the mobile station receiving this signal may experience random outages causing severe degradation in performance or loss of communication.
Mobile phone standards include a variety of equalization, data interleaving, and encoding techniques to compensate for signal fading. These are helpful but not sufficient to completely solve the problem. The power transmitted by the base station can usually be slightly increased through several approaches. A large power amplifier can be used to help overcome feeder losses, or tower mounted amplifiers or phased array antenna implementations (incorporating distributed amplifiers behind the elements) can boost the output power. However, the FCC places an upper limit on effective isotropic radiated power (EIRP). Even when the maximum legal power is achieved, current low noise amplifier (LNA) technology is such that the received signal-to-noise ratio at the base station is still better than that available at the mobile.
The forward link signal fading can be mitigated by dividing the signal between two or more antennas that are spatially separated and/or orthogonally polarized, so that multiple, uncorrelated signals reach the mobile receiver, as is done on the reverse link. However, if these signals are subsequently re-combined non-coherently at the receiver, a net loss in received signal strength is realized instead of a gain. All of the multiple transmission techniques used to date have employed some combination of alternating transmissions, pilot signals, additional channel coding, or multiple time slots as a means of maintaining separation of the two received signals within a single mobile receiver so that they can be coherently summed to provide signal gain. An intentional time delay can also be introduced between two signals transmitted to the mobile to introduce some inter-symbol interference (ISI) in the received signal at the mobile. This ISI enables an improvement in performance for receivers equipped with equalizer structure (found in nearly all mobile stations) because equalizers coherently combine two signals arriving through uncorrelated paths, yielding an improvement in performance. The delay is typically accomplished in the base station by dividing the baseband signal, introducing the delay in one path, and then utilizing two transmitters with redundant coaxial feeders.
In accordance with the present invention, a tower mounted antenna system for a wireless communication system has improved forward link performance that substantially improves or matches the reverse link performance thereby resulting in a balanced link at a greater distance from the base station than heretofore achievable. Improved performance is achieved through the integration of a diversity technique that improves the signal received at the mobile with diversity techniques that improve the signal received at the base station to xe2x80x9cbalancexe2x80x9d the RF link at a greater distance than possible using presently known techniques.
Polarity and time delay diversity schemes have been individually utilized in previous systems. There are also systems that include the fusion of polarity and time delay diversity techniques on transmit to provide an improvement in forward link performance that is greater than the sum of the techniques taken individually. This combination allows the exploitation of under-utilized RF channel compensation structures present in existing mobile receivers. Both Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) systems experience improvement since each standard utilizes a channel compensation technique (e.g. equalizers in TDMA and rake receivers in CDMA). Transmit delay diversity, as conventionally implemented, only extracts a small additional signal gain from the equalizer or rake receiver of the mobile receiver by introducing some inter-symbol interference (ISI). However, the delay that maximizes the gain from the equalizer or rake receiver (a time measured in microseconds) is much too short to provide any immunity from fast signal Rayleigh fading (time in milliseconds).
The present invention is an antenna system that provides multiple signal diversity in a unitary package and combines a time delayed version of the transmit signal with an orthogonal polarized antenna to provide a second uncorrelated, delayed signal that is coherently added by the channel compensation techniques of the mobile unit, simultaneously adding ISI and combating fast signal fading. Improved performance is experienced at the mobile, but effected from the base station antenna system. Furthermore, this particular implementation combines receive polarization, transmit and delay diversities with active antenna architectures to enable the maximum limit of RF power to be employed while simultaneously providing a balanced link at either a greater distance from the base station or deeper into buildings than is currently possible with the same base station locations. The improved system performance increases data transmission rates, or improves resource utilization (e.g. by reducing CDMA soft handoff requirements).
In accordance with the present invention, there is provided a communication system having a base station tower transmitting and receiving RF signals from at least one mobile station. The communication system comprises:
a first receive antenna (array) of a first polarization supported on the base station tower;
a second receive antenna (array) of a second polarization diverse to the first polarization supported on the base station tower;
a first transmit antenna (array) of a first polarization supported on the base station tower;
a second transmit antenna (array) of a second polarization diverse to the first polarization supported on the base station tower;
a first RF power amplifier(s) close coupled to the first transmit antenna on the tower;
a second RF power amplifier(s) close coupled to the second transmit antenna on the tower;
a first low noise amplifier(s) close coupled to the first receive antenna on the tower;
a second low noise amplifier(s) close coupled to the second receive antenna on the tower; and
an RF delay network close coupled to the second power amplifier(s) on the tower to generate primary and delayed orthogonal versions of a transmit signal from the first and second transmit antenna(s)(arrays).